The 18-million-share gift to the foundation is becoming an annual tradition for the philanthropic duo, who were also the second-most generous donors of 2012. In December of that year, the couple also gave 18 million shares to the foundation, though that gift was valued at around $500 million as the company's stock was trading at $27.71.

How the new funds will be put to use remains unknown, but both Zuckerberg and Chan have shown a predilection for sponsoring education- and healthcare-related programs. In January of this year, the couple earmarked $5 million from their previous gift to the Silicon Valley Community Foundation to help the Ravenswood Family Health Center build a new health center in East Palo Alto, Calif.

"Providing high quality healthcare for everyone is important for building a strong community," Zuckerberg said at the time.

The Breakthrough Prize in Life Sciences, which metes out million-dollar prizes to scientists working to extend human life, is also partially financed by Zuckerberg's contributions to the Silicon Valley Community Foundation.